Electrolytic case hardening



July 18, 1967 T. J. BUITKUS 3,331,708

ELECTROLYTIC CASE HARDENI NG Filed March 25, 1964 INVENTOR THOMAS J. BUITKUS United States Patent 3,331,708 ELECTROLYTIC CASE HARDENING Thomas J. Buitlrus, 838 Lenox Ave., Waukegan, Ill. 60085 Filed Mar. 23, 1964, Ser. No. 353,786 3 Claims. (Cl. 148-155) This invention relates to an electrolytic method of case hardening ferrous metal together with a means for regenerating the carburizing bath. More specifically, it relates to an electrocarburizing process in which ferrous metal articles are case hardened by being made the anode in an electrolytic cell containing a molten cyanogen compound electrolyte and a porous cathode electrode through which a cyanogen compound is introduced during the electrolysis for the purpose of regenerating the carburizing constituents of the electrolyte bath.

It is an object of this invention to provide an improved method of case hardening which is not dependent upon activating substances such as barium, calcium, and strontium salts employed in conventional carburizing baths to increase the rate of carburization. It is a further object of the present invention to provide a means for continuously regenerating the carburizing bath during the case hardening operation. Still another object is the provision of a liquid carburizing bath that will not leave an insoluble coating on the carburized article after it is quenched.

The present invention provides a process of producing case hardening in a relatively short period of time as compared with the current practice. In conventional liquid carburizing baths the carburizing action depends upon the conversion of cy-anides to cyanamides or cyanates to provide the carbon necessary for case hardening. In the present invention, case hardening is effected by the electrolytic decomposition of a molten cyanogen compound which results in the uniform distribution of nascent carbon in reactive contact with the articles being carburized. It is a further advantage of the invention that the controlled dissociation of the cyanide radical is localized at the surface of the articles being carburized thus assuring more effective use of the liberated active carbon.

The present invention will be described with reference to the accompanying drawing which is a diagrammatic plan view illustrating the basic components of the electrocarburizing apparatus, various parts associated therewith being shown in cross section.

Referring to the drawing, the numeral 1 designates a container or pot provided with a refractory lining 2 and filled with a molten carburizing electrolyte 3. The electrolyte 3 is maintained in the molten state by a source of heat represented by the electrical heating element 4 imbedded in a refractory casing 5. The ferrous metal article to be carburized, represented by a spur gear 6, is connected electrically by conductor 7 to the positive terminal of a source of direct current represented by rec tifier 8, the electrical output of which is controlled by variable transformer 9. The cathode electrode 10 comprises a hollow member having walls of an inert, porous, electrically conductive material which is connected electrically to the negative terminal of the direct current source by conductor 11 through a solenoid coil 12. An ammeter 13 and voltmeter 14 indicate the current and voltage of the electrical system. The solenoid coil 12 is provided with a movable plunger 15 which is attached to one end of a rack bar 16, the opposite end of said rack bar being attached to a fixed support 17 by an extension spring 18 provided with a tension adjustment screw 19. The opposite end of plunger 15 is likewise attached to a fixed support 20 by an extension spring 21 also provided with a tension adjustment screw 22. In engagement with the teeth of rack bar 16 is a spur gear 23 attached to the 3,331,708 Patented July 18, 1967 stem 24 of valve 25 which controls the flow of a cyanogen compound from cylinder 26 through pipes 27 and 28 into the interior of the cathode electrode 10.

In operation, the ferrous article 6 to be carburized is immersed in the electrolyte bath 3 until it reaches the proper temperature, preferably between 1200 F. to 1600 F. A direct current potential of 3 to.5 volts at a current density of 5 to 15 amp/sq. ft. or higher is then applied which causes electrolysis in which the cyanide anions (CN) present in the electrolyte 3 are attracted to the anode (spur gear) 6 and are neutralized. The neutralized cyanide radicals liberate nascent carbon and nitrogen per the equation:

The liberated carbon and nitrogen are absorbed by the item 6 being carburized and difiuse inwardly from the surface. The reaction involving the transfer of carbon occurs at the interface of the article being hardened and the molten electrolyte. A portion of the nitrogen liberated is also absorbed, the percentage varying with the temperature of the electrolyte. Small articles such as pins and bolts maybe batch carburized in accordance with the present invention by being immersed in a perforated container provided with anode contacts and tumbled during electrolysis in a manner similar to barrel electroplating.

The electric current flowing through solenoid coil 12 creates a magnetic field which draws plunger 15 inwardly which in turn moves rack bar 16 axially against the tension exerted by springs 18 and 21. The movement of rack bar 16 rotates gear 23 and stem 24 thereby opening valve 25 allowing the cyanogen compound from cylinder 26 to flow into the cathode electrode 10. The concomitant reduction of the metallic cations at the cathode 10 results in the deposition of free metal upon said electrode which combines chemically with the cyanogen compound entering through the porous walls to reform the carburizing constituents of the electrolyte. Since the temperature at which the reformation takes place is above the melting point of the reformed cyanogen compound, said compound is reformed in the liquid state and is quickly absorbed into electrolyte 3. The cyanogen compound consumed in the carburization process is thus regenerated at the cathode electrode 10 and returned to the electrolyte 3 to react anew.

As the current is increased, the magnetic flux in solenoid 12 increases proportionately drawing plunger 15 further inwardly thereby opening valve 25 wider and allowing more of the cyanogen compound from cylinder 26 to enter electrode 10. Conversely, as the current is decreased, the magnetic flux decreases in the solenoid and the tension of spring 18 imparted to rack bar 16 closes valve 25. The amount of the cyanogen compound released into cathode 10 is thus automatically balanced with the amount of metal set free at said cathode in order that all of the regenerant will be consumed and none will remain to escape from the cell to create a hazard to health or safety. Increasing the current density will increase the rate of carburization to a point after which a further increase will cause erosion of the anode and therefore is to be avoided. The processing time will vary depending on the nature of the material being hardened, the depth of case hardening desired, the composition and temperature of the electrolyte and the voltage and current density employed. The refractory lining 2 is preferably constructed of ceramic or other electrically insulating material to prevent stray electrical currents from originating between the walls of the container 1 and the anode 6 and cathode 10.

The composition of the electrolyte 3 is not critical, the prime requisite being that it contains a cyanogen com- 7 pound which under the influence of electrolysis will yield carbon at the anode 6 and a cyanogen combinable reduction product at the cathode 10. Examples of suitable compounds include sodium and potassium cyanide and other alkali metal cyanides, cyanates, cyanamides and the like. Thepercentage of the cyanogencompound in the electrolyte 3 may be varied from 100 percent to 30 percent or less, the remainder of the electrolyte being made up of various diluents such as alkali metal hydroxides and carbonates. The following examples will The cyanogen regenerant should be a compound capable of reacting chemically with the reduction product at the cathode to regenerate the carburizing constituents of the electrolyte 3. Such a compound shouldbe amenable to introduction into the electrolyte via the cathode electrode 10. Cyanogen, hydrogen cyanide, cyanic acid, and molten cyanamide and cyanuric acid are examples of suitable cyanogen compounds.

The electrocarburizing process may be started without the simultaneous introduction of the cyanogen regenerant into the cathode 10, but prolonged absence of said 'regenerant will cause the cyanide anion content of the -electrolyte to'become depleted and also will result in an accumulation of the'reduction product around the cathode electrode. Regeneration of the electrolyte in the manner disclosed herein will serve to maintain the proper composition of the carburizing bath except for the addition of reactants necessary to compensate for drag-out losses and thermal decomposition.

While this invention has been described with particular reference to the construction shown in the drawing and while various changes may be made in the detail construction and selection of reactants, it shall be understood that such changes shall be within the scope of the present invention as defined by the appended claims. h

What is claimed as new and desired to be protected'by Letters Patent is: i

1. The method of case hardening ferrous metal which comprises making said metal the anode electrode in an electrolytic cell containing a cathode electrode and a molten electrolyte, said electrolyte containing an alkali metal cyanogen compound as its essential ingredient, subjectingsaid anode to electrolysis in said electrolyte while simultaneous ly controlling by an electromagnetically actuated valve responsive to variations in anode current density the flow through said cathode electrode of a cyano compound selected from the group consisting of cyanogen, hydrogen cyanide, cyanic acid, cyanamide, and cyanuric acid. 7

2. The method of case hardening ferrous metal which comprises makin'g'said metal the anode electrode in an electrolytic cell containing a porous cathode electrode and a molten electrolyte, said electrolyte containing an alkali metal cyanogen compound as its essential ingredient, subjecting said anode to electrolysis in said electrolyte while simultaneously controlling by 'an.electro magnetically actuated valve'responsive to variationsin anode current density the flow through said cathode elec trode of a cyano compound selected from the group consisting of cyanogen, hydrogen cyaide, cyanic acid, cyanamide, and cyanuric acid, the amount of the cyano compound being sufficient to react with all of the reduction products liberated at said cathode electrode.

3. The method of case hardening ferrous metal which comprises making said'm'etal the anode electrode in an electrolytic cell containing a cathode electrode and a molten electrolyte, said electrolyte containing an alkali metal cyanogen compound as its essential ingredient, subjecting said anode to electrolysis in said. electrolyte while simultaneously controlling by a valve actuated by a solenoid responsive to variations in current density at the anode the flow through said cathode of a cyano type compound selected from the group consisting'of cyanogen, hydrogen cyanide, cyanic acid, cyanamide, and cyanuric acid, the amount of the cyano type compound being sufficient to reactwith all of the reduction products being liberated at said cathode electrode.

References Cited UNITED STATES PATENTS 1,837,070 12/1931 Roth 204 245 X 1,953,647 4/1934 Darrah 14815.5 2,041,769 5/1936 Larkin 148-15.5 X 2,273,795 2/1942 Heise et al. 204277 X 2,773,025 12/1956 Ricks et al. 20%277 X FOREIGN PATENTS 460,365 10/1949 Canada.

JOHN H. MA CK, Primary Examiner. G. KAPLAN, Assistant Examiner. 

1. THE METHOD OF CASE HARDENING FERROUS METAL WHICH COMPRISES MAKING SAID METAL THE ANODE ELECTRODE IN AN ELECTROLYTIC CELL CONTAINING A CATHODE ELECTRODE AND A MOLTEN ELECTROLYTE, SAID ELECTROLYTE CONTAINING AN ALKALI METAL CYANOGEN COMPOUND AS ITS ESSENTIAL INGREDIENT, SUBJECTING SAID ANODE TO ELECTROLYSIS IN SAID ELECTROLYTE WHILE SIMULTANEOUSLY CONTROLLING BY AN ELECTROMAGNETICALLY ACTUATED 